An apparatus and method for compressing fluid

ABSTRACT

The invention provides an apparatus for compressing a first fluid. The apparatus comprises a compressor piston comprising a piston cylinder and a piston assembly slidably mounted therein. The piston assembly comprises first and second spaced apart piston members defining a space therebetween. The space is configured to contain a second fluid used to cause compression of the first fluid. The piston assembly further comprises means for feeding second fluid to the space between the first and second piston members.

The present invention relates to an apparatus for compressing fluid. Inparticular, the invention relates to compressors, and particularly,although not exclusively, to oscillating compressors, and especially tohydraulically driven compressors. The invention relates to pistoncompressors or ionic compressors, and either single-stage or multi-stagecompressors. The invention primarily relates to seals within suchcompressors, in particular axial seals, which can be executed both asstem seals or piston seals. The invention further extends to methods ofcompressing fluids, and especially gas.

The gas seal in a compressor that is fitted with leakage relief issubjected to the full gas pressure during the compression process(equivalent to the actual gas-pressure), which will inevitably lead towear during continued use. Furthermore, with rising back-pressure on theseal within the compressor, wear on the seal also increases as a resultof elevated pre-stresses. Experience has shown that the service life ofa gas seal subjected to such stresses, given adequate dimensions, is inthe range of 2500-3000 km.

There is therefore a need to provide improved compressor designs, whichincrease the service life of the seals. The present invention arisesfrom the inventor's work in trying to overcome the problems associatedwith the prior art.

In accordance with a first aspect of the invention, there is provided anapparatus for compressing a first fluid, the apparatus comprising acompressor piston comprising a piston cylinder and a piston assemblyslidably mounted therein, wherein the piston assembly comprises firstand second spaced apart piston members defining a space therebetween,which space is configured to contain a second fluid used to causecompression of the first fluid, and means for feeding second fluid tothe space between the first and second piston members.

In prior art compressors, the fluid seal in contact with the compressedfluid is exposed to a pressure equivalent to the actual gas-pressure,which causes significant wear on the seal. However, in contrast, in theapparatus of the invention, the piston assembly comprises two spacedapart piston members, resulting in the fluid seal in the compressorbeing exposed to a reduced pressure of only 2 bar. Thus, the apparatusresults in a significant reduction of load on the fluid seal, leading toreduced wear and tear. Hence, advantageously, the apparatus results inextended wear time for the piston seals, due to excellent lubricationcreated between the piston assembly and the cylinder tube. This providesenhanced corrosion protection, and mechanical protection againstcompressor knocking, and results in low noise emission. Furtheradvantages include longer service lives leading to lower maintenancecosts, which results in increased plant availability.

Preferably, the apparatus comprises a storage tank configured to storethe second fluid therein. Preferably, the means for feeding the secondfluid to the space between the first and second piston members comprisesa pump, and preferably at least one second fluid feed conduit extendingbetween the storage tank and the space between the piston members alongwhich the fluid is fed.

Preferably, the first piston member (referred to herein as a “floatingpiston”) is configured to oscillate within the cylinder tube, and ispreferably sealed with the cylinder tube by a first radial seal. Thefirst radial seal may be a stem seal or a piston seal. Preferably, thefirst piston member is substantially centrally mounted on the secondpiston member, and is guided concentrically thereby. Preferably, thesecond piston member (referred to herein as a “main piston”) isconfigured to oscillate within the cylinder tube, and is preferablysealed with the cylinder tube by a second radial seal. The second radialseal may be a stem seal or a piston seal.

Preferably, the first fluid that is to be compressed contacts one sideof the first piston member, and the second fluid contacts the oppositeside of the first piston member. Preferably, the second fluid acts as alubricating fluid when disposed in between the first and second pistonmembers of the piston assembly, as it acts to reduce friction betweenthe first radial seal and the cylinder tube. Preferably, the secondfluid acts as a driving fluid when disposed underneath the pistonassembly, as it serves to cause the piston assembly to oscillate withinthe cylinder tube, thereby compressing the first fluid.

Preferably, the second fluid disposed in the space between the first andsecond piston members is fluidly connected, preferably via at least onesecond fluid leakage conduit, to the storage tank. Hence, any of thesecond fluid which leaks through the second seal is fed to the storagetank. Advantageously, therefore, the apparatus comprises a leakage cyclereturn line to the space between the first and second piston members,because, during use of the compressor piston, any leakage of the secondfluid at the second seal is automatically balanced out by areplenishment flow of second fluid.

Preferably, the second piston member comprises a valve configured tocontrol the flow of the second fluid through the at least one secondfluid feed conduit into the space between the piston members.Preferably, the valve comprises biasing means configured to bias thevalve into a closed configuration in the second fluid feed conduit. Thebiasing means preferably comprises a spring, and more preferably ahelical spring or a cup spring.

Preferably, the second piston member comprises actuation meansconfigured to activate the valve in response to a change in pressure onthe first seal, or in response to the position of the first pistonmember with respect to an actuation set-point. Preferably, the actuationmeans is configured to activate the pump in response to a change inpressure on the first seal, or in response to the position of the firstpiston member with respect to an actuation set-point. Preferably, theactuation means is configured to open the valve when the pressure on thefirst seal increases, and preferably activates the pump to pump secondfluid through the valve. Conversely, preferably the actuation means isconfigured to close the valve when the pressure on the first sealdecreases, and preferably deactivate the pump to prevent pumping ofsecond fluid.

By way of example, therefore, when the pressure on the first fluid sideof the first seal increases due to leakage of the second fluid throughthe second seal, the first piston member is preferably configured to beurged towards the second piston member, thereby resulting in theactuation means opening the valve.

In another preferred embodiment, the actuation means is configured toopen the valve when the position of the first piston member reaches theactuation set-point and/or the actuation means is configured to closethe valve when the position of the first piston member moves beyond theactuation set-point. It will be appreciated that the valve and pump maybe activated by the actuation means, when the position of the floatingpiston falls below the actuating set-point, which can occur due to fluidlosses, or entrapped compressible gases. This may not necessarily resultin a decrease or increase of pressure on the first seal.

Preferably, the pump is configured to pump the second fluid from thestorage tank, through the open valve activated by the actuation means,and along one or more conduit into the space between the first andsecond piston members. Preferably, the second piston member comprisesone or more conduits which extend radially outwardly from the valve tothe space between the piston members. Preferably, the one or moreconduits extend diagonally from the valve to the space between thepiston members.

Advantageously, a substantially constant depth of the second fluid ismaintained between the first and second piston members. The second fluidmay be pumped into the space between the piston members at any stage inthe compression process. However, preferably the pump is activatedduring the non-compression stage, i.e. when the piston assembly isdisposed at, or adjacent to, the bottom of the cylinder tube.

Preferably, the apparatus is configured such that the pressure betweenthe first fluid side of the first piston member and the second fluid issubstantially balanced out. Preferably, the pre-stress tension of thebiasing means exerted on the actuating means substantially correspondsto the weight of the first piston member and the friction createdbetween the cylinder tube and the first seal.

Preferably, the pressure difference between the side of the first pistonmember contacting the first fluid, and the side contacting the secondfluid is less than 75 Bar, more preferably less than 50 Bar, even morepreferably less than 25 Bar, and still more preferably less than 15 Bar.More preferably, the pressure difference between the side of the firstpiston member contacting the first fluid, and the side contacting thesecond fluid is less than 10 Bar, preferably less than 5 Bar, and mostpreferably less than 3 Bar.

As a result, small radial forces between the first piston member and thefirst radial seal results in less wear and tear. Advantageously, theapparatus is configured to subject the first seal only to thepre-tension pressure that is defined by the seal, in order to minimisewear thereon.

Preferably, the compressor piston comprises an inlet through whichuncompressed first fluid is fed therein, and an outlet through whichcompressed first fluid exits. Preferably, the pressure of the inletfluid is about 1-200 barg; more preferably about 1-30 barg; and mostpreferably about 3-10 barg.

Preferably, the compressor piston is configured to increase the pressureof the first fluid to between 100 bara and 1500 bara. More preferably,the compressor piston is configured to increase the pressure of thefirst fluid to between 150 bara and 1250 bara. Most preferably, thecompressor piston is configured to increase the pressure of the firstfluid to between 300 bara and 1000 bara. Preferably, the pressure of theoutlet fluid is about 350 Bar.

It may be appreciated that the desired pressure of the first fluidvaries depending upon the first fluid that is used. Accordingly, whenthe first fluid is hydrogen, the compressor piston may be configured toincrease the pressure of the first fluid to between 500 bara and 1500bara, more preferably to between 700 bara and 1400 bara, and mostpreferably to between 800 bara and 1300 bara.

Alternatively, when the first fluid is natural gas, the compressorpiston may be configured to increase the pressure of the gas to between100 bara and 700 bara, more preferably to between 200 bara and 600 bara,and most preferably to between 300 bara and 500 bara.

The first fluid may comprise liquid. Preferably, however, the firstfluid comprises gas, such as natural gas, fuel gas, hydrogen, gaseoushydrocarbon, liquefied combustion gas, nitrogen, helium, oxygen, and anoble gas, such as argon, or a mixture thereof. More preferably, thefirst fluid comprises a fuel gas, for example natural gas or hydrogen.

The second fluid may comprise liquid, which is preferably substantiallyincompressible. Preferably, the second fluid comprises an ionic liquid,an LOHC (liquid organic hydrogen carrier), semi-heavy water (HDO),deuterium oxide (heavy water), water, or hydraulic oil, or a mixturethereof. Most preferably, the second fluid comprises an LOHC or an ionicliquid. An ionic liquid consists substantially exclusively of ions, andis a class of material that is liquid at temperatures below 100° C.LOHCs are carbon-based liquids with very similar properties to ionicliquids. Advantages of ionic liquids and LOHCs are that they exhibit lowor no vapor pressure, good lubricating properties, essentially no gassolubility, high thermal stability, and high heat capacity.

In one embodiment, and preferably embodiments in which the second fluidis an ionic liquid, the apparatus is configured to use an ionic liquidcushion disposed between the piston assembly and the first fluid to becompressed. Preferably, the ionic liquid cushion is disposed on top ofthe first (i.e. floating piston member) and fills out all of the deadspace whilst in the compression phase. The ionic liquid cushionpreferably comprises a fluid with a low vapour pressure, and maycomprise or consist of a substantially pure ionic liquid, or a mixtureof an ionic liquid and LOHC.

Preferably, the apparatus comprises an oscillating compressor.Preferably, the apparatus comprises a hydraulically driven compressor.In one preferred embodiment, the apparatus comprises a pistoncompressor. Preferably, the apparatus comprises a liquid pistoncompressor, in which the second fluid (preferably a liquid) is used todrive compression of the first fluid (preferably a gas). In anotherpreferred embodiment, the apparatus comprises an ionic compressor.

In one embodiment, the apparatus comprises a single-stage compressor.Preferably, the apparatus comprises a plunger functionally connected toone or more displacement pistons configured to oscillate within ahousing, and configured to displace the second fluid to and from thecompressor piston, thereby compressing the first fluid therein. Thedisplacement pistons may be connected in series. Oscillation of the oreach displacement piston driven by the plunger is facilitated by alubricant which is fed into the housing via at least one inlet. In someembodiments, the lubricant for the plunger may be hydraulic oil, LOHC oran ionic liquid, or mixtures thereof.

In another embodiment, the apparatus preferably comprises a multi-stagecompressor (e.g. 2-stage, 3-stage or 4-stage) comprising a plurality ofcompressor stages connected in a series. Preferably, the apparatuscomprises between one and twenty compressor stages. More preferably, theapparatus comprises between two and ten compressor stages. Mostpreferably, the apparatus comprises between three and five compressorstages. In a most preferred embodiment, the apparatus comprises fourcompressor stages connected in series.

The apparatus may comprise a multistage compressor comprising aplurality of compressor stages connected in parallel. Advantageously,this would increase the throughput of the compressor.

Accordingly, in one embodiment, the apparatus may comprise a pluralityof series, wherein each series comprises a plurality of compressorstages connected in a series and the plurality of series are connectedparallel.

According to a second aspect of the invention, there is provided amethod of compressing a first fluid, the method comprising:

-   -   feeding a first fluid into a compressor piston comprising a        piston cylinder and a piston assembly slidably mounted therein,        wherein the piston assembly comprises first and second spaced        apart piston members defining a space therebetween, which space        is configured to contain a second fluid used to cause        compression of the first fluid; and    -   feeding a second fluid to the space between the first and second        piston members, and compressing the first fluid.

Preferably, the method of the second aspect comprises use of theapparatus of the first aspect.

Preferably, the method comprises pumping the second fluid to the spacebetween the first and second piston members, preferably along at leastone second fluid feed conduit extending between a second fluid storagetank and the space between the piston members.

Preferably, the method comprises feeding any of the second fluid, whichleaks through a second radial seal disposed between the second pistonmember and the cylinder tube, to the storage tank.

Preferably, the method comprises controlling the flow of the secondfluid through the at least one second fluid feed conduit into the spacebetween the piston members via a valve disposed in the at least onesecond fluid feed conduit. Preferably, the method comprises biasing thevalve into a closed configuration in the second fluid feed conduit. Thebiasing means preferably comprises a spring, and more preferably ahelical spring.

Preferably, the method comprises activating the valve in response to achange in pressure on the first radial seal, or in response to theposition of the first piston member with respect to an actuationset-point. Preferably, the method comprises activating the pump inresponse to a change in pressure on the first radial seal, or inresponse to the position of the first piston member with respect to anactuation set-point. Preferably, the method comprises opening the valvewhen the pressure on the first radial seal increases, and preferablypumping the second fluid through the valve. Preferably, the methodcomprises closing the valve when the pressure on the first radial sealdecreases, and preferably deactivating the pump to prevent pumping ofsecond fluid.

In use, when the pressure on the first fluid side of the first radialseal increases due to leakage of the second fluid through the secondradial seal, the method comprises urging the first piston member towardsthe second piston member, thereby resulting in the opening of the valve.

The method may comprise opening the valve when the position of the firstpiston member reaches the actuation set-point and/or closing the valvewhen the position of the first piston member moves beyond the actuationset-point.

Preferably, the method comprises pumping the second fluid from thestorage tank, through the open valve, and along one or more conduit intothe space between the first and second piston members. Preferably, themethod comprises pumping second fluid along one or more conduits whichextend radially outwardly from the valve to the space between the pistonmembers.

Preferably, the method comprises maintaining a substantially constantdepth of the second fluid between the first and second piston members.The method may comprise pumping the second fluid into the space betweenthe piston members at any stage in the compression process. However,preferably the method comprises activating the pump during thenon-compression stage, i.e. when the piston assembly is disposed at, oradjacent to, the bottom of the cylinder tube.

Preferably, the method comprises balancing the pressure between thefirst fluid side of the first piston member and the second fluid.Preferably, the pressure difference between the side of the first pistonmember contacting the first fluid, and the side contacting the secondfluid is less than 75 Bar, more preferably less than 50 Bar, even morepreferably less than 25 Bar, and still more preferably less than 15 Bar.More preferably, the pressure difference between the side of the firstpiston member contacting the first fluid, and the side contacting thesecond fluid is less than 10 Bar, preferably less than 5 Bar, and mostpreferably less than 3 Bar.

Preferably, the method comprises feeding uncompressed first fluid intothe compressor piston via an inlet, and feeding compressed fluid throughan outlet. Preferably, the method comprises use of a liquid pistoncompressor, in which the second fluid (preferably a liquid) is used todrive compression of the first fluid (preferably a gas). In anotherpreferred embodiment, the apparatus comprises an ionic compressor.

Preferably, the method comprises displacing the second fluid to and fromthe compressor piston, thereby compressing the first fluid therein, bymeans of one or more displacement pistons configured to oscillate withina housing.

The first fluid may comprise liquid. Preferably, the first fluidcomprises gas, such as natural gas, fuel gas, hydrogen, gaseoushydrocarbon, liquefied combustion gas, nitrogen, helium, oxygen, and anoble gas, such as argon, or a mixture thereof.

The second fluid may comprise liquid, which is preferably substantiallyincompressible. Preferably, the second fluid comprises an ionic liquid,an LOHC (liquid organic hydrogen carrier), semiheavy water (HDO),deuterium oxide (heavy water), water, or hydraulic oil, or a mixturethereof. Most preferably, the second fluid comprises an LOHC or an ionicliquid.

In one embodiment, and preferably embodiments in which the second fluidis an ionic liquid, the method comprises use of an ionic liquid cushiondisposed between the piston assembly and the first fluid to becompressed. Preferably, the ionic liquid cushion is disposed on top ofthe first piston member, and fills out all of the dead space whilst inthe compression phase. The ionic liquid cushion preferably comprises afluid with a low vapour pressure, and may comprise or consist of asubstantially pure ionic liquid, or a mixture of an ionic liquid andLOHC.

All features described herein (including any accompanying claims,abstract and drawings), and/or all of the steps of any method or processso disclosed, may be combined with any of the above aspects in anycombination, except combinations where at least some of such featuresand/or steps are mutually exclusive.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a first embodiment of a gas compressoraccording to the invention having two spaced apart piston compressors(left and right-hand sides) each one having a piston assembly, which isslidably mounted in a cylinder tube;

FIG. 2 is a schematic diagram of a second embodiment of the gascompressor according to the invention having two spaced apart pistoncompressors (left and right-hand sides) each having a slidably mountedpiston assembly. The piston assembly in each compressor uses an ionicliquid cushion thereon, and the piston assembly of the left-hand pistoncompressor has moved to the top of its cylinder tube, therebycompressing gas therein via the ionic cushion, and the piston assemblyof the right-hand piston compressor is positioned towards the middle ofits cylinder tube, such that the gas remains substantially uncompressed;

FIG. 3 is a cross-sectional side view of the compressor shown in FIG. 1,in which the piston assembly of the left-hand piston compressor hasmoved to the top of its cylinder tube, thereby compressing gas therein,and the piston assembly of the right-hand piston compressor ispositioned at the base of its cylinder tube, such that the gas remainsuncompressed. Fresh gas is sucked in towards the bottom dead centre;

FIG. 4 is an enlarged cross-sectional side view of the top of theleft-hand piston compressor shown in FIG. 3 with the piston assemblypositioned at the top of its cylinder tube having compressed the gas;and

FIG. 5 is an enlarged cross-sectional side view of one piston assemblyof a piston compressor present in the compressor of the invention.

EXAMPLE

Referring to FIGS. 1-3, there are shown embodiments of a compressor 2for compressing gas 14, such as natural gas (CNG), fuel gas, hydrogen,gaseous hydrocarbons, liquefied combustion gas, nitrogen, helium,oxygen, and noble gases such as argon. For example, the compressor 2 canbe used to compress hydrogen which is used as fuel in hydrogen-drivenvehicles. Compression is hydraulically driven, for example by means ofan ionic compressor or by a piston compressor, as shown in the Figures.It will be appreciated therefore that the compressor 2 is a liquidpiston compressor.

FIGS. 1 and 2 show first and second embodiments of the compressor 2,respectively. In each embodiment, the compressor 2 includes two, spacedapart piston compressors 4 in parallel, into which uncompressed gas 14is fed via an inlet 40, and from which compressed gas 14 exits viaoutlet 41. The pressure of the inlet gas 14 is about 6 Bar, and thepressure of the outlet, compressed gas 14 is about 350 Bar. The inlet 40and outlet 41 are fitted with multichannel valves 44 with very lowfrequency expectation values (a compressor frequency 0.1 Hz-5 Hz, morepreferably 0.5 Hz-1.5 Hz means low actuating frequencies for the valvesas well) to allow the passage of gas 14 therethrough.

As can be seen in the Figures, the illustrated compressor 2 is asingle-stage compressor (i.e. 1-stage). The piston compressors 4 in theillustrated 1-stage system are in parallel and are driven by a singleplunger 30 which causes the reciprocal oscillation of pistons 32connected thereto within a housing 58. Each piston 32 is connected to acorresponding pump 42, which is arranged to displace hydraulic drivingfluid 16 disposed in a reservoir 60 to and from its correspondingcompressor piston 4, thereby compressing the gas 14 therein.

However, multi-stage compressors are also envisaged in which at leasttwo of the compressors 2 in FIG. 3 are connected in series, so that thedischarge through outlet 41 of both same pressure stage compressors 4are connected to the suction inlet port 40 of the higher pressure stage.For example, there may be four compressor 2 stages, in which thepressure of the inlet gas 14 into the first compressor 2 is 6 bara, andthe pressure of the outlet, compressed gas 14 is 16.6 bara; the pressureof the inlet gas 14 into the second compressor 2 is 16.6 bara, and thepressure of the outlet gas 14 is 45.7 bara; the pressure of the inletgas 14 into the third compressor 2 is 45.7 bara, and the pressure of theoutlet gas 14 is 126 bara; and the pressure of the inlet gas 14 into thefourth compressor 2 is 126 bara, and the pressure of the outlet gas 14is 350 bara.

The hydraulic driving fluid 16 is incompressible, and can be any ionicliquid, an LOHC (liquid organic hydrogen carrier), heavy water,deuterium oxide, water, or hydraulic oil, or mixtures thereof. Theoverall hydraulic system needs to be designed for the lower lubricity ofthe heavy water, for example, compared to a standard lubricant like oil.Oscillation of the pistons 32 driven by the plunger 30 is facilitated bya lubricant 34 which is fed into the housing 58 via inlets 54. In someembodiments, the lubricant 34 for the plunger 30 may be hydraulic oil34, LOHC or an ionic liquid, or mixtures thereof. The lubricant 34should be kept separate from the driving fluid 16 because it needs tohave a different compression ratio.

In FIGS. 2 and 3, the compressor 2 is shown with its left-hand pistoncompressor 4 in a configuration such that it is compressing the gas 14,and with its right-hand piston compressor 4 in a configuration in whichgas 14 remains substantially uncompressed, after fresh gas has beensucked in through suction valve 40. Position sensors 46 connected toeach pump 42 detect the configuration of each piston compressor 4, andfacilitate respective oscillations therein, such that gas 14 isautomatically fed into the piston compressors 4 through inlets 40, andcompressed, and then expelled at high pressure through outlets 41.

In prior art compressors, gas seals disposed the gas being compressed 14and the pistons are subjected to the full gas pressure, which lead towear during continued use. However, referring to FIGS. 3 and 4, thecompressor 2 of the invention is fitted with a mechanism by which thelife-time of gas seals 18 within the piston compressors 4 issignificantly extended by reducing wear and tear thereon. As can be seenmost clearly in FIG. 5, each piston compressor 4 includes a cylindertube 6 in which a piston assembly 7 (also known as a “dummy” piston) isslidably mounted. Each piston assembly 7 consists of a floating piston10 connected to a spaced apart main piston 8. The floating piston 10 isarranged to oscillate within the cylinder tube 6, and is sealed thereinby a radial gas seal 18, such as a V-piston ring. One side of thefloating piston 10 (i.e. the upper side shown in FIGS. 1, 2 and 5) is incontact with the gas 14 that is to be compressed (e.g. hydrogen, orcompressed natural gas, CNG). On its opposite side (i.e. the lower sideshown in FIGS. 1, 2 and 5), the floating piston 10 is in contact with athin layer of the same incompressible hydraulic driving fluid 16, whichis displaced by pistons 32 to cause the piston assembly 7 to oscillatewithin the cyclinder tube 6.

The floating piston 10 is centrally embedded within the main piston 8,and is guided concentrically thereby. The main piston 8 is also slidablymounted within the cylinder tube 6 and is sealed therewith by a radialhydraulic seal 20, such as a V-piston ring. The incompressible hydraulicdriving fluid 16 disposed in the space between the floating piston 10and the main piston 8 is fluidly connected, via a duct 26 along whichany leaked hydraulic fluid through seal 20 is fed, to a storage tank 28in which replenishment hydraulic driving fluid 16 is stored, which isshown in FIGS. 1 and 2.

Referring to FIG. 5, the storage tank 28 creates a leakage cycle returnline to the space between the pistons 8, 10, because, during use of thecompressor piston 4, any leak of hydraulic driving fluid 16 at thehydraulic seal 20 can be automatically balanced out by a replenishmentflow of driving fluid 16, as follows. The main piston 8 has a hydraulicfluid replenishment feed valve 24, which is fluidly connected byconduits 38, 50 to the storage tank 28. The valve 24 is biased into aclosed position by a helical spring 22 or a cup spring 22 actingthereon. However, if the pressure on the gas side of the first seal 18increases due to leakage of driving fluid 16 through seal 20, thefloating piston 10 is urged towards the main piston 8, resulting in thereplenishment feed system being activated via an actuating unit 12connected to the valve 24. The valve 24 is opened by the actuating unit12, and hydraulic fluid 16 is pumped by pump 48 from the storage tank 28along conduits 50, 38, through the open valve 24, and along diagonalconduits 36, which lead directly into the space between the main piston8 and the floating piston 10. Accordingly, a constant depth of hydraulicdriving fluid 16 is maintained between the floating piston 10 and mainpiston 8. The replacement driving fluid 16 can be pumped back into thespace between the floating piston 10 and main piston 8 at any stage inthe process. However, in the embodiment shown in the Figures, the pump48 is activated when the piston assembly 7 is disposed at the bottom ofthe cylinder tube 6, i.e. the non-compression stage.

The pressure between the gas side of the floating piston 10 and theincompressible hydraulic fluid 16 is designed to be constantly balancedout. The spring's 22 pre-stress tension on the actuating unit 12corresponds to the weight of the floating piston 10 and the frictioncreated between the cylinder tube 6 and the gas seal 18. The pressuredifference between the side of the floating piston 10 contacting the gas14, and the side contacting the hydraulic fluid 16 is less than 2 Bar,and small radial forces between the floating piston 10 and the seal 18results in less wear and tear.

The system described above therefore always attempts to subject thefirst gas seal 18 only to the pre-tension pressure that is defined bythe seal 18, in order to minimise wear on the seal 18. In prior artcompressors, the gas seal in contact with the compressed gas 14 isexposed to a pressure equivalent to the gas pressure, which causes wear,whereas by splitting the piston assembly 7 into two (i.e. the floatingpiston 10 and the main piston 8), the gas seal 18 in the compressor 2 ofthe invention is exposed to a reduced pressure of only 2 bar. Hence, theinvention results in the significant reduction of load on the gas seal18. Although the hydraulic seal 20 is exposed to the similar pressuresto that experienced in the prior art compressor, it does not affect thesystem as a whole because any leakage of hydraulic fluid 16 isimmediately re-injected back into the space between the pistons 10, 12along conduits 36 from storage tank 28.

The embodiment of the compressor 2 shown in FIG. 3 is essentially thesame as that shown in FIG. 2 except that, in FIG. 3, an ionic liquidcushion 56 is provided in between the piston assembly 7 and the gas 14being compressed. This is useful in embodiments when the hydraulicdriving fluid 16 is itself an ionic liquid, and may not be necessarywhen the driving fluid 16 is an LOHC. The ionic liquid cushion 56 is ontop of the floating piston 10 and fills out all of the dead space whilstin the compression phase. The ionic liquid cushion 56 comprises a fluidwith a low vapour pressure, and can be made up of any pure ionic liquid,or a mixture of ionic liquid and LOHC.

Advantages of the compressor 2 reside in extended wear time for thepiston seals 18, 20 (>20.000 h), due to very good lubrication createdbetween the pistons 8, 10 and the cylinder tube 6. This providesexcellent corrosion protection, and mechanical protection againstcompressor knocking, and so results in low noise emission. Furtheradvantages include longer service lives leading to lower maintenancecosts. This results in increases plant availability, and lowerrequirements on the opposite contact face because of lower contactpressure forces, which again, minimises maintenance costs.

1. An apparatus for compressing a first fluid, the apparatus comprisinga compressor piston comprising a piston cylinder and a piston assemblyslidably mounted therein, wherein the piston assembly comprises firstand second spaced apart piston members defining a space therebetween,which space is configured to contain a second fluid used to causecompression of the first fluid, and means for feeding second fluid tothe space between the first and second piston members.
 2. An apparatusaccording to claim 1, wherein the apparatus comprises a storage tankconfigured to store the second fluid therein, and the means for feedingthe second fluid to the space between the first and second pistonmembers comprises a pump and at least one second fluid feed conduitextending between the storage tank and the space between the pistonmembers along which the fluid is fed, preferably wherein the pump isactivated during the non-compression stage and the second fluid disposedin the space between the first and second piston members is fluidlyconnected to the storage tank via at least one second fluid leakageconduit.
 3. An apparatus according to claim 2, wherein the second pistonmember comprises a valve configured to control the flow of the secondfluid through the at least one second fluid feed conduit into the spacebetween the piston members, preferably wherein the valve comprisesbiasing means configured to bias the valve into a closed configurationin the second fluid feed conduit, more preferably wherein the biasingmeans comprises a spring, optionally a helical spring or a cup spring.4. An apparatus according to claim 3, wherein the second piston membercomprises actuation means configured to activate the valve in responseto a change in pressure on the first seal or in response to the positionof the first piston member with respect to an actuation set-point,preferably wherein the pump is configured to pump the second fluid fromthe storage tank, through the open valve activated by the actuationmeans, and along one or more conduit into the space between the firstand second piston members.
 5. An apparatus according to claim 4, whereinthe actuation means is configured to open the valve when the pressure onthe first seal increases, and activate the pump to pump second fluidthrough the valve and/or the actuation means is configured to close thevalve when the pressure on the first seal decreases, and deactivate thepump to prevent pumping of second fluid, and/or wherein when thepressure on the first fluid side of the first seal increases due toleakage of the second fluid through the second seal, the first pistonmember is configured to be urged towards the second piston member,thereby resulting in the actuation means opening the valve, and/orwherein the pre-stress tension of the biasing means exerted on theactuating means substantially corresponds to the weight of the firstpiston member and the friction created between the cylinder tube and thefirst seal.
 6. An apparatus according to claim 4, wherein the actuationmeans is configured to open the valve when the position of the firstpiston member reaches the actuation set-point and/or the actuation meansis configured to close the valve when the position of the first pistonmember moves beyond the actuation set-point.
 7. An apparatus accordingto claim 1, wherein the first piston member is configured to oscillatewithin the cylinder tube, and is sealed with the cylinder tube by afirst radial seal, optionally a stem seal or a piston seal, and thesecond piston member is configured to oscillate within the cylindertube, and is sealed with the cylinder tube by a second radial seal,optionally a stem seal or a piston seal, and the first piston member issubstantially centrally mounted on the second piston member, and isguided concentrically thereby.
 8. An apparatus according to claim 1,wherein the first fluid that is to be compressed contacts one side ofthe first piston member, and the second fluid contacts the opposite sideof the first piston member, preferably wherein the apparatus comprises aleakage cycle return line to the space between the first and secondpiston members, because, during use of the compressor piston, anyleakage of the second fluid at the second seal is automatically balancedout by a replenishment flow of second fluid.
 9. An apparatus accordingto claim 1, wherein the second piston member comprises one or moreconduits which extend radially outwardly from the valve to the spacebetween the piston members, preferably wherein the one or more conduitsextend diagonally from the valve to the space between the pistonmembers.
 10. An apparatus according to claim 1, wherein the pressuredifference between the side of the first piston member contacting thefirst fluid, and the side contacting the second fluid is less than 75Bar, 50 Bar, 25 Bar, 15 Bar, 10 Bar, 5 Bar, or less than 3 Bar and thecompressor piston is configured to increase the pressure of the firstfluid to between 100 bara and 1500 bara.
 11. An apparatus according toclaim 1, wherein the first fluid comprises gas, such as natural gas,fuel gas, hydrogen, gaseous hydrocarbon, liquefied combustion gas,nitrogen, helium, oxygen, and a noble gas, such as argon, or a mixturethereof and the second fluid comprises liquid, which is substantiallyincompressible, preferably wherein the second fluid comprises an ionicliquid, an LOHC (liquid organic hydrogen carrier), semiheavy water(HDO), deuterium oxide (heavy water), water, or hydraulic oil, or amixture thereof.
 12. An apparatus according to claim 1, wherein theapparatus is configured to use an ionic liquid cushion disposed betweenthe piston assembly and the first fluid to be compressed, preferablywherein the ionic liquid cushion comprises or consists of asubstantially pure ionic liquid, or a mixture of an ionic liquid andLOHC.
 13. An apparatus according to claim 1, wherein the apparatuscomprises: an oscillating compressor and/or a hydraulically drivencompressor; and/or a liquid piston compressor and/or an ioniccompressor; and/or a single-stage compressor or a multi-stagecompressor; and/or a plunger functionally connected to one or moredisplacement pistons configured to oscillate within a housing, andconfigured to displace the second fluid to and from the compressorpiston, thereby compressing the first fluid therein, preferably whereinoscillation of the or each displacement piston driven by the plunger isfacilitated by a lubricant which is fed into the housing via at leastone inlet, wherein the lubricant is hydraulic oil, LOHC or an ionicliquid, or mixtures thereof.
 14. A method of compressing a first fluid,the method comprising: feeding a first fluid into a compressor pistoncomprising a piston cylinder and a piston assembly slidably mountedtherein, wherein the piston assembly comprises first and second spacedapart piston members defining a space therebetween, which space isconfigured to contain a second fluid used to cause compression of thefirst fluid; and feeding a second fluid to the space between the firstand second piston members, and compressing the first fluid.
 15. A methodaccording to claim 14, wherein the method uses an apparatus comprising acompressor piston comprising a piston cylinder and a piston assemblyslidably mounted therein, wherein the piston assembly comprises firstand second spaced apart piston members defining a space therebetween,which space is configured to contain a second fluid used to causecompression of the first fluid, and means for feeding second fluid tothe space between the first and second piston members.